I know I’m going to be nervous and not be at my best when I audition – what should I do?

Energy spent on worrying is better spent on practicing!
Remember that the only thing you have control over is your audition preparation – focus on that instead of the things out of your control (such as how others sing).
Replace negative/anxious thoughts with positive ones – instead of thinking that you’ll feel horribly nervous, think about how relaxed and excited you will feel to be able to sing for people.
Have family and friends help you prepare by walking into a room and singing for them as you would in an audition. Doing this will teach you how to switch into performance mode quickly.
Drink water! You will feel better!